The present invention relates generally to chemical-mechanical planarization apparatus used in manufacturing semiconductors and more specifically to an improved polishing pad that allows in situ monitoring of a wafer during a chemical-mechanical polishing process.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness.
Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing lithographic process steps that create integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modem complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithographic processing steps. Poor optical resolution prohibits the printing of high density features. Another problem with nonplanar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar interconnect surface layers are required in the fabrication of modem high-density integrated circuits. To this end, CMP tools have been developed to provide controlled planarization of both structured and unstructured wafers.
CMP consists of a chemical process and a mechanical process acting together, for example, to reduce height variations across a dielectric region, clear metal deposits in damascene processes or remove excess oxide in shallow trench isolation fabrication. The chemical-mechanical process is achieved with a liquid medium containing chemicals that react with the front surface of the wafer when it is mechanically stressed during the planarization process.
In a conventional CMP tool for planarizing a wafer, a wafer is secured in a carrier connected to a shaft. The shaft is typically connected to mechanical means for transporting the wafer between a load or unload station, and a position adjacent to a polishing pad mounted to a rigid or flexible platen. Pressure is exerted on the back surface of the wafer by the carrier in order to press the wafer against the polishing pad, usually in the presence of slurry. The wafer and/or polishing pad are then moved in relation to each other via motor(s) connected to the shaft and/or platen in order to remove material in a planar manner from the front surface of the wafer.
It is often desirable to monitor the front surface of the wafer during the planarization process (in situ). One known method is to use an optical system that interrogates the front surface of the wafer in situ by positioning an optical probe under the polishing surface and transmitting and receiving the optical signal through an opening in the polishing pad. In some systems the opening in the polishing pad is filled with an optically transparent material, or xe2x80x9cwindowxe2x80x9d, in order to prevent polishing slurry or other contaminants from being deposited into the probe and obscuring the optical path to the wafer. It is possible to adjust the planarization process based upon these real-time measurements or to terminate the process once the front surface of the wafer has reached a desired condition. However, several problems exist with current window technology. One such problem is that separations start to form at the surfaces between the window and the polishing pad when the polishing pad is stressed during the planarization process of the wafer. Even extremely small separations are undesirable as contamination can accumulate within the separations and scratch the front surface of the wafer or cause optical interference. Scratching and optical interference can also result from abrasive particles becoming trapped in the window material itself, or from the surface of the window projecting above the surrounding pad material. Another problem is that the optical clarity of the pad window can be degraded due to the presence of trapped gas bubbles within the window material. Still other problems include chemical degradation, staining, and poor optical clarity of the window. In addition, some windows may undesirably absorb some of the UV light spectra that offers the most ideal signal response in the monitoring process.
What is needed is a polishing pad window (or lens) that does not scratch the wafer, is chemically and stain resistant to the polishing environment, and that has good optical properties through which in situ CMP monitoring may be undertaken.
The present invention provides an improved polishing pad window for use during a planarization process of a front surface of a wafer and a method of manufacturing the window.
The method of manufacturing the polish pad window may include a degassing step in which the window material, polish pad or both are degassed in order to reduce the size and number of unwanted gas bubbles in the window, thereby improving the optical qualities of the window. The degassing may be accomplished by warming the window material and/or polishing pad or by using known vacuum techniques to release absorbed gasses in the window material and/or polishing pad.
A hole is created in the polishing pad at every location where a window is desired. The method for creating the hole is not critical, but should produce a hole with minimal fuzz or loose particles on the walls of the hole. The method for creating the holes is preferably able to accurately position the holes on the polishing pad without damaging or contaminating the polishing pad.
The process may be further improved by positioning a first release film against the working surface of the polishing pad over the hole. The first release film prevents the later inserted optically clear window material from draining out the hole and forms a top surface for the window.
If it is desired for the top surface of the window to be lower than the working surface of the polishing pad, a plug may be placed into the hole. This prevents the window material from becoming coplanar with the working surface of the polishing pad thereby recessing the top surface of the window. This may be necessary if the window material would scratch or damage the front surface of the wafer if the wafer touched the window.
The working surface of the polishing pad is preferably turned face down before the hole is filled with an optically clear window material. With the working surface face down, bubbles that form will travel away from the eventual top surface of the window and any adverse effects will be minimized. The optically clear window material is preferably curable with UV light to quickly set and bond the window into the hole.
Another improvement to the process is to position a second release film against the bottom surface of the polishing pad over the hole. The second release film helps to create a bottom surface for the window that is coplanar with the bottom surface of the polishing pad.
After the window has been cured and the first and second release films have been removed as needed, the top and bottom surface of the window may be conditioned to remove excess window material. In addition, it may be desirable to condition the window""s surfaces so that its top and bottom surface are coplanar with the corresponding working surface and bottom surface of the polishing pad.
A polishing pad with a cured window cast in place may thus be created.